1. Field of the Invention
The present invention relates to a rolling bearing which is suited to use for supporting rotation in machinery requiring silence in {circle around (1+L )} use for supporting rotation of information machinery such as hard disc drive (HDD), video tape recorder (VTR), digital audio tape recorder (DAT), {circle around (2+L )} use for supporting a swinging part of a swing arm as a composing part of HDD, and {circle around (3+L )} motors for fans or cleaners, or turbocharger for vehicle.
2. Description of the Related Art
The rolling bearing for the above usage is required to be low torque and good acoustic characteristic (low noise), and therefore composing parts of bearings as an inner ring, an outer ring or rolling elements are finish-processed at dimensionally high precision. The inner, outer rings and the rolling elements are fabricated with high carbon chromium bearing steels as SUJ2 or martensite based stainless steels as SUS440C, and then performed with hardeningxe2x80x94tempering, so that hardness of a raceway surface is to be HRC58 to 64.
Information machinery has recently been miniaturized and often portably served, and accordingly so much exposed to risks of receiving shocks by dropping or vibrations comparing with occasions in a past. Accompanied therewith, the rolling bearings within the machinery are much probable to be injured. In a small sized ball bearing employed in portable information machinery, since a contact ellipse generated in a contacting face between a bearing ring and rolling elements is small, if adding an impact load, the contacting part is permanently deformed even if the impact load is relatively small, and the raceway surface is probably impressed with depression (impression or indentation), resulting in deteriorating an acoustic characteristic or causing irregularity in rotation torque.
As examples of the conventional arts for solving the above problem, technologies described in JP-A-7-103241 and JP-A-8-312651 may be enumerated.
According to JP-A-7-103241, the amount of a residual austenite in a steel serving as the raceway surface is decreased to be 6 vol % or less so as to heighten an impression resistance in the raceway surface, whereby the raceway surface is prevented from a permanent deformation when the rolling bearing receives the impact load. For example, the bearing ring is formed with SUJ2, and thereafter subjected to a hardening at standard temperature therefor (820 to 860xc2x0 C.), and to a sub-zero treatment or a tempering treatment at relatively high temperature as 220 to 240xc2x0 C., thereby reducing the amount of the residual austenite as low as possible while keeping a hardness requisite to the raceway surface.
In JP-A-8-312651, for heightening the impression resistance in the raceway surface, it is described to form the bearing ring with an ordinary bearing steel (a case-hardening steel, or high carbon chromium bearding steels as SUJL to 3), then perform a carbonitriding hardening treatment and the tempering at 350xc2x0 C. or higher, thereby decreasing the amount of the residual austenite to be 0% in a steel for forming the raceway surface. It is further described to form the bearing ring with a steel which is added to SUJ2 with an element for imparting resistance against a temper softening, and perform a quench hardening and the tempering at 350xc2x0 C. or higher, thereby decreasing the amount of the residual austenite to be 0% in a steel forming the raceway surface. It is also set forth to make the rolling elements of a ceramic, thereby to avoid the rolling elements from the impression by contacting with the raceway surface.
However, the above conventional technologies do not pay attention to the acoustic characteristic when being exposed to vibration, and a room is still left for also improving the acoustic characteristic when receiving impact load. That is, there is a problem that, in the rolling bearing within the machinery, fine vibrations or fluctuations given to the machinery cause fretting (a phenomenon where two faces to be contacted repeatedly cause a relative fine sliding, and thus become worn) in the contacting faces of the rolling elements and the bearing ring to deteriorate the acoustic characteristic. However, the above publications make no reference thereto.
The invention has been realized in view of problems involved with the conventional art, and accordingly it is an object of the invention to provide a rolling bearing suited to a small sized and portable information machinery, and excellent in the acoustic characteristic.
For solving the above problems, the rolling bearing according to the invention which is provided, as a bearing ring, with an outer ring and an inner ring or a shaft when a raceway surface of the inner ring is formed in the shaft, is characterized in that, of the inner ring (or the shaft) and an outer ring, at least the inner ring (or the shaft) is fabricated with a bearing steel of carbon being 0.6 wt % or higher and has a carbonitrided layer in the raceway face thereof, and a multiple Gd of an impression resistance expressed with the following formula (1) is 1.6 or more by nitrogen content percentage (wt %): Np at a position (a position of 2% Da depth) to be from the surface of the raceway to a side of a core portion by the amount of a dimension corresponding to 2% of a diameter of a rolling element and by Vickers hardness (Hv): H0.2 at said position of 2% Da depth.
Gd=(H0.2/500)+2xc2x7Npxe2x80x83xe2x80x83(1). 
In the rolling bearing of the invention which is provided, as the bearing ring, with the outer ring and the inner ring or the shaft that the raceway surface of the inner ring is formed in the shaft, it is preferable that at least the inner ring (or the shaft) of the inner ring (or the shaft) and an outer ring is fabricated with a bearing steel of carbon being 0.6 wt % or higher, has a carbonitrided layer in the raceway face thereof, and at the position (the position of 2% Da depth) from the surface of the raceway to the side of the core portion by the amount of a dimension corresponding to 2% of the diameter of the rolling element, the amount of a residual austenite is 1.0 vol % or lower, the hardness at the above-mentioned position (the position of 2% Da depth) is HRC58 or more in Rockwell hardness (Hv653 or more in Vickers hardness), the nitrogen content at the above-mentioned position is 0.05 wt % or higher, and the multiple Gd of the impression resistance expressed with the following formula (1) by the nitrogen content percentage (wt %): Np and by Vickers hardness (Hv): H0.2 at the above-mentioned position is 1.6 or more.
Gd=(H0.2/500)+2xc2x7Npxe2x80x83xe2x80x83(1). 
Inventors found that even if the carbonitrided layer formed in the raceway surface has the same hardness, the higher the nitrogen content in the carbonitrided layer, the better the impression resistance. A book entitled xe2x80x9cElastic coefficient of metal materialsxe2x80x9d (October 1980) issued by Japan Machinery Society, mentions at page 11 xe2x80x9cit is considered that with respect to the elastic coefficient of an alloy in solid solution, in case a solute atom is an interstitial atom, crystal lattice is largely disturbed and the elastic coefficient is inevitably lowered.xe2x80x9d As mentioned, it is assumed that even if the hardness is the same, the higher the nitrogen content, the lower the elastic coefficient in the carbonitrided layer. That is, although the carbonitrided layer of the same hardness is formed in the raceway surface, if nitrogen content is higher in the carbonitriding layer, an elastic deformability is higher in the raceway surface, so that the contacting surface pressure with the rolling elements is small and the impression is less to occur.
The invention specifies the nitrogen content to be 0.05 wt % or higher in a position where a maximum shearing stress in a depth direction of the carbonitrided layer is added (such a position to be from the raceway surface to the side of the core portion by the amount of the dimension corresponding to 2% of the diameter of the rolling element). With this structure, and an elastic deformability requisite to an excellent impression resistance is given to the carbonitrided layer.
By specifying the nitrogen content to be 0.05 wt % or higher in the above-mentioned position in the carbonitrided layer, an abrasion amount by an adhesion abrasion is reduced at the contact surface between the rolling element and the bearing ring, and an abrasion resistance is increased. Accompanied therewith, as the contacting face between the rolling element and the bearing ring is difficult to generate fretting injuries, in case fine vibration or fluctuation is added, a deteriorating degree in the acoustic characteristic of the rolling bearing can be lessened. It is preferable that the nitrogen content at the above-mentioned position is 0.1 wt % or higher. If the nitrogen content is higher, the impact resistance and the fretting resistance are increased, and so, more preferably 0.2 wt %, but in contrast, as a grindability is worsened, preferable is 0.6 wt % or lower as seen in A-10 of Table 4.
The multiple Gd of the impression resistance expressed with the formula (1) shows a total value of the hardness in the raceway surface+the elastic deformability, and if the Gd is 1.6 or higher, an excellent impression resistance (impact resistance) is obtained. The higher the more preferable the multiple Gd of the impression resistance, and in particular, although an upper limit in the performance is not specified, the higher the multiple Gd of the impression resistance, the higher costs to be required for materials or thermal treatments. Therefore, the multiple Gd of the impression resistance should be determined in relation with the costs. If the Gd is large, a grinding time is lengthened, and preferable is 2.7 or less as seen in A-10 of Table 4.
The invention specifies the hardness at the depth position of the raceway surface (the position to be from the raceway surface to the side of the core portion by the amount of the dimension corresponding to 2% of the diameter of the rolling element) to be HRC58 or more in Rockwell hardness (Hv653 or more in Vickers hardness). Thereby, the hardness in the raceway surface necessary as the bearing can be held. The hardness in the raceway surface at the depth position is preferably HRC60 (Hv697) or more.
In the invention, of the inner ring and an outer ring, at least the inner ring is fabricated with a bearing steel of carbon being 0.6 wt % or higher. Herein, the bearing steel is meant by an iron-steel material having a composition which can exhibit a performance requisite as the bearing. As such bearing steels, listed are, for example, high carbon chromium bearing steels of SUJL to SUJ5, SUS440C, 13Cr-martensite based stainless steel, or SCR420.
The bearing ring fabricated with such a bearing steel containing carbon 0.6 wt % or higher is carried out with the carbonitriding treatment, followed by tempering at e.g., 250 to 320xc2x0 C., thereby enabling to specify the amount of the residual austenite to be 1.0 vol % or lower at the position (of 2% Da depth) to be from the raceway surface to the side of the core portion by the amount of the dimension corresponding to 2% of the diameter of the rolling element, and specify the hardness in the raceway surface and the hardness at the depth position to be HRC58 or more.
Further, it is preferable that the sub-zero treatment is carried out after the carbonitriding treatment, so that the residual austenite is induction-transformed to decrease the amount of the residual austenite, and subsequently the tempering is practiced. When the residual austenite is dissolved as much as possible before tempering by practicing the sub-zero treatment before the tempering, the lattice of the martensite structure is much disordered after the carbonitriding treatment, and the lowering action of the elastic coefficient caused by the carbonitriding treatment as mentioned above is accelerated.
In the invention, it is sufficient that of the inner ring and the outer ring, at least the inner ring has the above mentioned structure, and preferably each of the inner and outer rings has such a structure.
The invention does not make a special limitation to the rolling elements, and allows any sorts made of the conventionally used SUJ2, ceramic, or stainless. In case the rolling element is made of ceramic, the fretting resistance is by far improved comparing with metal-made rolling elements, but in case the bearing ring is as conventional, since a large surface contacting pressure between the raceway surface and the ceramic-made rolling elements is generated, the impression resistance in the raceway surface is lowered.
On the other hand, in the rolling bearing of the invention, since the bearing ring is fabricated as above mentioned, the impression resistance in the raceway surface can be kept high, in spite of the ceramic-made rolling elements. Accordingly, in the rolling bearing of the invention, while the impression resistance in the raceway surface can be kept high by using the ceramic-made rolling elements, a higher fretting resistance can be provided.
As the rolling element composing the rolling bearing of the invention, it is desirable that the rolling element is fabricated with a stainless steel containing Cr 8 wt % or higher (preferably, 12 wt % or higher), a nitride layer is formed in the surface by a nitriding treatment, and a core portion contains nitrogen and carbon contents of 0.45 wt % or higher in a total amount. Thus, by containing nitrogen and carbon contents of 0.45 wt % or higher in the total amount, the core portion is avoided from softening by tempering at high temperature after the nitriding treatment.
If the nitride forming elements such as Mo, V, W, Nb, Al, Si or others are added in said stainless steel, fine nitrides thereof are formed to more heighten durability of the rolling element, and it is preferable to properly add these elements in view of costs.
It is preferable that a surface layer of the rolling element has anitrided layer of Vickers hardness (Hv) being 1000 or more. Further, as a combination of the rolling elements and the bearing ring (the inner ring or the shaft having raceway surface of the inner ring, and the outer ring), it is preferable that a difference between the surface hardness of the rolling element and the hardness of a truck surface of the bearing ring or at the position of 2%Da depth, is Hv300 or higher, more preferably Hv500 or more. Thereby, the fretting resistance is particularly made excellent, and the impact resistance (the impression resistance) is maintained high.